1. Field of the Invention
The present invention relates to a lightweight composite ceramic with low heat conductivity and high fracture toughness, and a process for producing the same.
2. Description of the Prior Art
Conventional methods wherein an oxide is added to silicon (Si) and the resulting mixture is reaction-sintered for the purpose of promoting nitriding of Si and enhancing the strength of the resulting sinter are disclosed in, e.g., Japanese Patent Laid-Open No. 88,173/1983, Japanese Patent Laid-Open No. 152,271/1984, Japanese Patent Laid-Open No. 207,876/1984, Japanese Patent Laid-Open No. 207,877/1984, Japanese Patent Laid-Open No. 217,673/1984, Japanese Patent Laid-Open No. 186,470/1985, Japanese Patent Publication No. 34,677/1984, and Japanese Patent Publication No. 34,678/1984. Further, lowly heat-conductive ceramics already examined as materials for use in engines include aluminum titanate (Al.sub.2 TiO.sub.5) and partially stabilized zirconia (ZrO.sub.2), and some of them have been put into practical use.
Further, Japanese Patent Laid-Open No. 261,662/1991 discloses a ceramic composition and a process for producing a ceramic member therefrom. The above ceramic composition comprises a metallic Si powder or a mixed powder thereof with a ceramic powder, an organic binder added thereto in an amount of 6 to 25 wt. % based on the total weight of the composition, a deflocculant, and water. The above ceramic powder comprises at least one of Si.sub.3 N.sub.4, Al.sub.2 TiO.sub.5, mullite and potassium titanate, and is prepared through firing in an atmosphere of nitrogen.
Further, Japanese Patent Laid-Open No. 296,771/1990 discloses a composite ceramic and a process for producing the same. This composite ceramic is a dense sinter composed of composite particles formed from a material comprising a first ceramic and second ceramic particles dispersed therein and having a sintering temperature lower than that of the first ceramic. Herein, the second ceramic is lower in heat conductivity than the first ceramic.
However, the fact is that the aforementioned ceramic produced by adding an oxide to Si and reaction-sintering the resulting mixture for the purpose of promoting nitriding of Si and enhancing the strength thereof is not well lowered in heat conductivity. Further, Al.sub.2 TiO.sub.5 has a strength of at most 50 MPa and hence cannot be used as such as a structural ceramic for engines and the like, for which the above-mentioned strength is too low. Furthermore, ZrO.sub.2, though having a high strength, has a high thermal expansion coefficient to develop a high thermal stress disadvantageously.
Meanwhile, the quantity (Q) of heat transfer to a member in an unsteady state is represented by the formula: Q=(2..kappa..c..rho..T/.pi.).sup. 1/2 .times..DELTA..theta..sub.o .times.A, wherein A is the surface area, .DELTA..theta..sub.o is the temperature amplitude on the surface, T is the temperature on the surface, .kappa. is the heat conductivity, c is the heat capacity or specific heat, and .rho. is the density, or specific gravity.
As is understandable from the above formula, the product of heat conductivity (.kappa.), specific heat (c) and specific gravity (.rho.) must be small in an aspect of the properties of a material used in the member in order to decrease the quantity (Q) of heat transfer. However, the square root (.kappa..times.c.times..rho.).sup. 1/2 of the value of the above product is 5 for ZrO.sub.2 and 8.7 for Si.sub.3 N.sub.4. In view of the above, supposing that the heat conductivity (.kappa.), specific heat (c) and specific gravity (.upsilon.) of a material are low, for example, in the case of a lightweight and lowly heat-conductive material having a heat conductivity (.kappa.) of 2.1 W/m.K, a specific heat (c) of 0.7 J/g.K and a specific gravity (.rho.) of about 2.2, the value of the above square root becomes about 1.8, which means that the quantity (Q) of heat transfer can be decreased to about 36% of that of ZrO.sub.2 and about 20% of that of Si.sub.3 N.sub.4.
There is also known silicon nitride (Si.sub.3 N.sub.4) prepared as a low heat-conductive ceramic by dispersing ceramic whiskers in a matrix material and reaction-sintering them in order to improve the toughness thereof. Further, procedures of adding an oxide such as Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 to Si and reaction-sintering them with a view to enhancing the strength of the resulting sinter are disclosed in, e.g., Japanese Patent Laid-Open No. 152,271/1984 and Japanese Patent Laid-Open No. 207,876/1984.
Further, Japanese Patent Laid-Open No. 218,974/1991 discloses a silicon nitride sinter and a process for producing the same. This silicon nitride sinter has a microstructure comprising 20 to 50% of particles having a length of at least 10 .mu.m and 20 to 50 of particles having a length of at most 3 .mu.m, and has properties such as a heat conductivity of 0.13 to 0.16 cal/cm..degree.C and a four-point flexural strength at room temperature of at least 800 MPa.
However, the above low heat-conductive ceramic obtained by dispersing ceramic whiskers in a matrix material and reaction-sintering them has a fracture toughness (K.sub.IC) of about 4 to 5 MPa.m.sup. 1/2, and hardly brings about phonon scattering because of the absence of a different phase in the grain boundaries and hence is not sufficiently low in heat conductivity. Further, a target for improving the toughness of a ceramic has been a reaction product .beta.-Si.sub.3 N.sub.4. Thus, there have been no ceramics comprising an oxide dispersed in the matrix phase thereof for the purpose of lowering the heat conductivity thereof.